In the fields of image display devices, cathode ray tubes (CRTs) are being replaced with liquid crystal display devices (LCDs) and plasma display panels (PDPs).
LCDs are light and slim and have low power consumption. Also, LCDs do not radiate harmful electromagnetic waves. Because of these advantages, LCDs are considered as next-generation image display devices.
An LCD includes a bottom substrate, a top substrate, and a liquid crystal layer. A plurality of thin film transistors (TFTs) are formed in the bottom substrate, and color filters are arranged in the top substrate. The liquid crystal layer is interposed between the bottom substrate and the top substrate. The liquid crystal layer is formed by injecting liquid crystal between the two substrates.
The TFTs formed in the bottom substrate function to transmit and control electric signals. Also, molecular structure of liquid crystal is changed according to voltage applied thereto and thus light transmissivity is controlled. Through these procedures, the controlled light passes through the top substrate and are displayed as a desired image.
Meanwhile, since the LCD is a light receiving device that displays an image by controlling an amount of incident light, the LCD requires a backlight assembly that is a light source emitting light on a liquid crystal panel.
The backlight assembly is classified into a direct type backlight assembly and an edge type backlight assembly according to the arrangement of the light source. The direct type backlight assembly has high light utilization and is convenient to handle. Also, the direct type backlight assembly has no limitation in a screen size. Therefore, the direct type backlight assembly is widely used in large-sized LCDs.
External light sources for the backlight assembly can be appropriately selected according to size and purpose of the LCD. Examples of the external light sources include a point light source (e.g., an incandescent lamp, a white halogen lamp, etc.), a line light source (e.g., a hot cathode fluorescent lamp, a cold cathode fluorescent lamp, an external electrode fluorescent lamp, etc.), an electroluminescent (EL) lamp, and a surface light source (e.g. matrix type light emitting diodes (LED)).
With the large scale of display devices, various researches are conducted to improve light efficiency of the backlight assembly for large-sized display devices, for example 30 inches or more.